This invention relates to monolithic ceramic capacitors having base-metal electrodes and more particularly to such capacitors wherein the dielectric body is a reduction resistant glass-ceramic material having a low sintering temperature and a high dielectric constant.
Conventional monolithic ceramic capacitors are fired in air at a temperature of 1350.degree. C or above. These capacitors employ electrodes of noble metals such as platinum or palladium, sometimes alloyed with gold. During the past few years a growing number of monolithic capacitors have been produced that have a glass-ceramic dielectric body, that are capable of being fired in air and fully densified at temperatures generally below 1200.degree. C and that contain lower cost electrodes, usually of a palladiumsilver alloy. In addition, monolithic capacitors with even lower cost electrodes have been developed. These capacitors employ electrodes of a base metal such as Ni (sometimes referred to as b.m.e. monolithic capacitors).
Monolithic ceramic capacitors with base-metal electrodes, however, must be fired in an atmosphere of low oxygen pressure to prevent oxidation of the base metal which may render the base-metal non-conductive. Thus, the ceramic material must be reduction resistant to avoid becoming semiconductive or conductive during sintering in the low oxygen pressure atmosphere. Compared with monolithic ceramic capacitors having electrodes with noble metals, it is necessary in the manufacture of b.m.e. monolithic capacitors to provide more precise control over the stoichiometry of the ceramic formulation and to exercise tighter control over the firing conditions, both of which add cost in manufacturing.
Also, the glass-free ceramic materials normally used in such b.m.e. capacitors are particularly difficult if not impossible to sinter and fully densify at temperatures low enough to permit the use of copper electrodes. Copper melts at 1083.degree. C and copper electroded monolithic capacitors must be fired below this temperature. It is accordingly not known to manufacture such copper electroded monolithic capacitors even though the conductivity of copper is over 4 times greater than that of nickel, the usual electrode material.
Further, regarding ceramic dielectric materials containing glass, no known high dielectric constant glass-ceramic materials are believed to be capable of sintering in a low partial oxygen atmosphere without experiencing greatly degraded electrical properties. The term dielectric constant (K) as used herein refers to the room temperature value unless otherwise noted and the term high dielectric constant refers to values greater than 1000. All glass compositions known to have been used in a high dielectric constant glass-reacted-ceramic have included the oxides of lead, cadmium and/or bismuth, each of which are readily reduced when heated in a low partial oxygen atmosphere. The major adverse effect of reducing these oxides is to render the glass-ceramic dielectric material conductive.
Accordingly, it is an object of the present invention to provide a b.m.e. monolithic ceramic capacitor having a reduction resistant body of high dielectric constant capable of sintering and fully densifying at temperatures below 1080.degree. C.
It is a further object of the present invention to provide a monolithic ceramic capacitor having a buried copper electrode.
It is a further object of the present invention to provide a low cost monolithic capacitor having excellent electrical properties.